LIFTING POWER OF THE MOON. 43 



The popular explanation of the lifting power of the 

 moon on the waters of the ocean, is usually given in the 

 following terms. If, in the annexed diagram (fig. 2), 



FIG. 2. 



V 



o 



the moon be at M, and if A c B D be a great circle of the 

 earth, covered by a uniformly deep sea, the attractive 

 force of the moon will cause the waters vertically under 

 her at A to be heaped up, whilst the waters at c and D 

 will be drawn away or depressed. We shall thus have 

 high water at A, and low water at c and D. All this 

 seems reasonable enough ; but it is a puzzle to many 

 pei-sons why, on this theory, there should also be, as 

 we know from experience there is, high water at B, 

 the point most distant from the attracting body. A 

 moment's consideration will solve this difficulty ; for 

 we must consider that though the moon acts more 

 forcibly on the waters which are nearest to her than on 

 the centre of the earth, vertically beneath the point of 

 greatest attraction, yet she also acts more strongly on 

 the mass of the earth than on the waters which cover 

 the hemisphere most distant from her. The result is 

 that the waters rise on the surface facing the moon, 

 whilst the earth sinks (so to say) on the surface turned 

 from the moon. Thus, at each point, A and B, the 

 waters deepen, or, in other words, the tide rises. The 

 maximum lifting power exercised by the moon on the 



